Quantum teleportation, the transfer of unknown quantum state information, has been experimentally verified through a host of experiments. In addition, the key resource underpinning teleportation, quantum entanglement, has been experimentally verified over large ranges. An entanglement measurement over 144 km, achieved using optical free-space communications between two telescopes, proves the validity of ground-station to satellite quantum communications, and is a major step in the path towards a global quantum communications network. In such a network, a combination of satellite and fiber optic links would interconnect a multitude of quantum nodes, quantum devices and quantum computers. In optical fiber, transmission of entangled photons is limited to about 100 km by losses and de-coherence effects. Communications over fiber beyond this range would require use of either quantum repeaters, or the trusted relay paradigm used in a recent deployment of an eight-node quantum network.
Experimental verification of quantum superdense coding has also been achieved through a series of experiments. In superdense coding, two bits of classical information can be transferred at the cost of only one qubit.
Teleportation and superdense coding are strongly related and indeed are often considered as protocols that are the inverse of each other, differing only in how and when teleportation and superdense coding utilize quantum entanglement.
U.S. Pat. No. 7,362,420 B2 (U.S. Ser. No. 11/088,205) issued on 22 Apr. 2008 to Zaugg describes an entangled-photons range finding system and method. That is, the location of an object is found using quantum entanglement that is said to be substantially immune to detection by others. The method and system are directed to a new radar technique and is not applicable to quantum networks. The distance to an object is determined using simultaneously generated first and second photons. The first photon is reflected off an object, and the second photon is directed to an optical cavity. The arrival of the first photon is correlated with the arrival of the second photon, and the distance to the object is at least partially determined using the correlation.
However, the method and system of U.S. Pat. No. 7,362,420 do not find the location in an unconditional manner. In fact, the method and system can be spoofed. An adversary could readily spoof the method and system by using a photon detector to capture any entangled photons sent to the adversary, hold onto the sent entangled photons for awhile, and then send the entangled photons back to the sender. The sender would consequently find a location or position that is false. In fact, the adversary would be at a different position. In practice, distances cannot be measured unconditionally and as such, the method and system of U.S. Pat. No. 7,362,420 cannot verify and thus authenticate the locations of a device.
U.S. Pat. No. 7,075,438 B2 (U.S. Ser. No. 10/903,220) issued on 11 Jul. 2006 to Kent et al. describes tagging systems, and in particular a method of authenticating the position of a tagging device. A Bell pair comprising two photons is separated into first and second entangled particles, which are transmitted from first and second equidistant transmitter devices to the tagging device at a position relative to the transmitter devices and detector devices. The tagging device comprising a quantum gate determines response information by recombining the entangled particles and transmits a signal to the two detector devices, at least one of which records the arrival time of the signal at the receiving detector device. The transmitting devices and detector devices are connected to a management module. The receiving detector device is selected on the basis of the determined response information. The receiving detector device and the arrival time of the signal at the receiving detector device are compared with at least one expected receiving detector device and an expected arrival time of the signal for the expected receiving detector device. If the expected and actual signal arrival times for an expected detector device match, this verifies the position of the tagging device.
U.S. Pat. No. 7,075,438 teaches shining line-of-sight quantum beams of light onto the tagging device, which sets off an alarm if the tagging device is moved away from its position. The system of U.S. Pat. No. 7,075,438 cannot be deployed over a communication network where the sender cannot physically “see” the receiver (i.e. no line-of-sight path). Such a system therefore cannot verify the location of a device behind a wall, for example. Also, the system of U.S. Pat. No. 7,075,438 cannot be deployed over fiber-based communication networks. In emerging quantum networks, transport of quantum information occurs using photons transported over fiber. In a fiber-based communication network, the system of U.S. Pat. No. 7,075,438 can be readily spoofed due to the fact that light travels slower in fiber than air. Also, in most fiber connections, a straight-line fiber path between sender and receiver is unavailable. An adversary could exploit these facts by intercepting quantum signals on the fiber and communicating the quantum signals more quickly to other physical locations under its control using some form of wireless quantum communications alone, or classical wireless communications in conjunction with quantum teleportation. The system of U.S. Pat. No. 7,075,438 B2 is predicated on the need for quantum information to be transported via a line-of-sight path through air, thus rendering it of no value in verifying the location of a device in a communication network.